This invention relates generally to trigger circuits and in particular to a trigger circuit utilizing two comparators for generating a trigger signal in response to a fast electrical pulse commonly known as a glitch.
Measuring an electrical signal with an instrument such as a digital storage oscilloscope often requires obtaining a trigger signal to synchronize the measurement process with the event of interest. Such a trigger signal is usually generated by a separate internal trigger circuit in the instrument to initiate the instrument measurement sequence.
Triggering on a periodic waveform such as a sinusoidal 60 hertz electrical power line signal is easily done with a single comparator trigger circuit which generates a trigger circuit in response to the input signal voltage passing through a selected trigger voltage reference level. Choosing an appropriate trigger point on the power line signal is a matter of selecting an appropriate trigger voltage level.
Glitches are transient voltage waveforms that do not occur periodically. Glitches can be caused by random events, such as a lightning stroke or a switch closure. Glitch signals may be in the form of positive transient voltages, negative transient voltages, or bipolar (both positive and negative) transient voltages, depending on how they were generated. A trigger circuit is often necessary to synchronize the measurement equipment to the occurrence of the glitch since the glitch duration is often very short and its time of occurrence is not predictable. Assuming the type of glitch is known, a single comparator trigger circuit programmed with the proper trigger voltage level will allow an appropriate trigger signal to be generated, synchronizing the instrument measurement sequence with the desired transient event to be measured.
A common measurement task is to evaluate power line signals for noise superimposed on the power line signal by measuring the glitches as they occur. The periodic power line signal is combined with the nonperiodic glitch signal. Successfully measuring the glitch signal requires having a trigger signal responsive to the glitch signals in order to synchronize the measurement sequence. However, glitch signals that are superimposed on a sinusoidal signal present a significant problem for conventional trigger circuits when the amplitude of the glitch signal is less than that of the power line signal or the voltage polarity of the glitch is not known. The glitch signal voltage will very likely be masked in the power line signal voltage, thereby foiling a conventional trigger circuit which is responsive only to the input signal passing through a discrete voltage level of the input signal, resulting in a trigger signal that is not responsive to the glitches of interest. It is therefore desirable to have a glitch trigger circuit to provide a trigger signal responsive to glitch signals that are superimposed on sinusoidal power line signals, regardless of whether the glitches are bipolar, positive, or negative polarities and whether the glitch voltage level is less than the power line voltage level.